1. Field of the Invention
The present invention relates to methods and low density compositions for cementing pipe in well bores, and more particularly, to such methods and compositions for cementing pipe in well bores penetrating subterranean zones or formations which readily fracture at low hydrostatic pressures.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in oil, gas and water well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior of pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In some locations, the subterranean zones or formations into or through which wells are drilled have high permeabilities and low compressive and tensile strengths. As a result, the resistances of the zones or formations to shear are low and they have very low fracture gradients. When a well fluid such as a hydraulic cement composition is introduced into a well bore penetrating such a subterranean zone or formation, the hydrostatic pressure exerted on the walls of the well bore can exceed the fracture gradient of the zone or formation and cause fractures to be formed in the zone or formation into which the cement composition is lost.
While lightweight cement compositions have been developed and used heretofore, i.e., cement compositions having densities as low as about 12 pounds per gallon, subterranean zones or formations are still encountered which have fracture gradients too low for even the lightweight cement compositions to be utilized without fracturing the formation and the occurrence of lost circulation problems. Also, the lightweight cement compositions utilized heretofore have often not had sufficient compressive, tensile and bond strengths upon setting.
Thus, there are continuing needs for improved methods of cementing pipe in well bores and low density cement compositions which have enhanced compressive, tensile and bond strengths upon setting.